1. Field of the Invention
This invention relates to a fuel pump, and in particular to a spill pump which is capable of delivering a low minimum quantity of fuel.
2. Description of the Related Art
One example of a known fuel pump is a rotary spill pump for supplying fuel to a four cylinder engine which comprises a distributor member which is rotatable within a sleeve. The distributor member includes an enlarged region which extends out of the sleeve and is rotatable within a cam ring, the enlarged region being provided with four equi-angularly spaced, radially extending plunger bores within which respective plungers are reciprocable. The plungers engage respective shoe and roller arrangements which, in turn, engage the cam surface of the cam ring. As the distributor member rotates, the plungers slide within the bores under the influence of the cam surface.
The distributor member is provided with an axially extending passage which communicates with the plunger bores and with a delivery passage which is arranged to register with four delivery ports provided in the sleeve, in turn, upon rotation of the distributor member to permit delivery to the injectors of an associated engine in turn. The axially extending passage also communicates with an inlet arrangement whereby fuel is supplied, at relatively low pressure, to the plunger bores.
A spill valve communicates with the axially extending passage, the spill valve comprising a piston member carrying a valve member which is engageable with a seating defined around an end of the axially extending passage. The piston member is spring biased towards a position in which the valve member engages the seating. The spill valve is fuel pressure actuated, movement of the valve member occurring when high pressure fuel is applied to a surface of the piston member, the application of such high pressure fuel to the surface of the piston member being controlled by an electromagnetically actuable control valve.
In use, after the rollers have ridden over the crests of the cam lobes, fuel is supplied to the plunger bores pushing the plungers outwardly. When the plungers engage the leading flanks of the next cam lobes, the plungers are pushed inwardly, pressurising the fuel in the plunger bores and supplying fuel at high pressure to the delivery passage and the selected one of the delivery ports. After a predetermined time, and hence after delivery of a predetermined quantity of fuel, the control valve is opened to apply high pressure fuel to the spill valve piston member to open the spill valve permitting fuel to flow to a spill chamber. The flow of fuel to the spill chamber permits the fuel pressure in the axially extending passage to fall; hence, fuel delivery is subsequently terminated.
After the rollers ride over the crests of the cam lobes, the spring returns the spill valve member into engagement with its seating, returning the fuel from the spill chamber to the plunger bores which are supplemented by fuel from the inlet arrangement. The pumping cycle then continues, the rotation of the distributor member resulting in the high pressure fuel being delivered to a different one of the delivery ports.
As the spill valve opens under the action of the application of high pressure fuel to the piston member, the minimum quantity of fuel which can be delivered by the pump is determined, in part, by the length of the flow path from the plunger bores to the spill valve by way of the control valve. In use, by the time the pressure wave generated by the inward movement of the plungers has passed the control valve and reached the spill valve, a quantity of fuel has been delivered; thus, even if the control valve is left open to enable spill to occur as early as possible, some fuel will be delivered in each pumping cycle.
A number of other fuel pumps are known in which a pumping plunger is reciprocable under the influence of a cam surface. For example, in a known arrangement, the plunger is rotatable within a bore, and a cam surface is provided on an end of the plunger cooperating with a plurality of rollers to cause the plunger to reciprocate as it rotates. In another arrangement, a plurality of plungers are reciprocable under the influence of cam surfaces associated with a cam shaft, for example in an in-line pump, or where one or more unit injectors are used.